FAA Ground School Practice Test

If a flight transitions from an area of high pressure to an area of low pressure without adjusting the altimeter, what will the altimeter indicate?

1. Lower than the actual altitude above sea level

2. Higher than the actual altitude above sea level

3. Accurate altitude above sea level

4. Unable to determine altitude

The correct answer is: Higher than the actual altitude above sea level

When a flight transitions from a high-pressure area to a low-pressure area without adjusting the altimeter, the altimeter will indicate a higher altitude than the actual altitude above sea level. This occurs because altimeters are calibrated to reference standard atmospheric conditions, which include a specific pressure setting. In high-pressure zones, the air density is greater, which means that for a given altitude, the pressure is higher than standard atmospheric pressure. Consequently, if you do not adjust your altimeter when moving into a low-pressure area, the instrument maintains the higher pressure setting from the previous region. Thus, as the aircraft descends into the low-pressure area, the altimeter fails to register this decrease in pressure, leading it to reflect a greater altitude than what the aircraft is truly at. This situation can lead to dangerous scenarios such as flying at an erroneously high altitude, which may result in obstacles like terrain or other aircraft being closer than the altimeter indicates. Properly adjusting the altimeter to the current local pressure setting when transitioning between different pressure systems is essential for accurate altitude readings and safe flight operations.